The present invention relates to a hydraulic mechanism and particularly to a hydraulic power assist steering gear.
Hydraulic power assist steering gears are well known. Typically, such steering gears include a housing which defines a chamber. A piston divides the chamber into first and second chamber portions. The piston is movable in a work stroke within the chamber due to fluid pressure. A steering valve directs fluid into one of the chamber portions and vents the other chamber portion to effect movement of the piston in a given direction. An input shaft is rotated by the driver of the vehicle to effect actuation of the steering valve. The piston is drivingly connected to a vehicle steering mechanism to effect actuation thereof upon movement of the piston.
The vehicle steering mechanism has stops which limit the amount of movement of the steered wheels in both directions of turning. Typically, a stop is fixed to the axle and another stop is movable with the steered wheel and can engage the stop fixed to the axle to limit movement of the steered wheels in one direction. Similar stops engage to limit movement of the steered wheels in the opposite direction. The amount of movement of the steered wheels before the stops engage may vary from vehicle to vehicle because of manufacturing tolerance and other factors. The piston in the power steering gear moves through a work stroke, the length of which is proportioned to the amount of movement of the steered wheels before the stops engage.
A known steering gear includes relief valves which are carried by the piston. The relief valves, commonly known as end-of-stroke relief valves, allow fluid communication between the chamber portions on opposite sides of the piston as the piston approaches the end of its work stroke in either direction and just prior to engagement of the stops. The relief valves prevent high pressure build-up which could damage the steering gear.
The location of the piston when a relief valve opens is important because the relief valve is to open just prior to engagement of the steering mechanism stops. The relief valve is adjustable to change the location that the piston will reach prior to opening of the relief valve. Over the life of a relief valve, several adjustments may be necessary.